170 HUMAN PHYSIOLOGY. 



Function of the Tensor Tympani Muscle. The function of this 

 muscle is, as its name indicates, to increase the tension of the membrane in 

 accordance with the pitch of the sound wave. The tendon of this muscle 

 playing over the processus cochleariformis and attached at almost a right 

 angle to the handle of the malleus, will, when the muscle contracts, pull 

 the handle inwards, increase the convexity of the membrane, and at the 

 same time increase its tension ; with the relaxation of this muscle, the handle 

 of the malleus passes outward and the tension is diminished. The contractions 

 of the tensor muscle are reflex in character and excited by nerve impulses 

 reaching it through the small petrosal nerve and otic ganglion. The number 

 of nerve stimuli passing to the muscle and determining the degree of con- 

 traction will depend upon the pitch of the Sound wave and the subsequent 

 excitation of the auditory nerve. The tensor tympani muscle may be re- 

 garded as an accommodative apparatus by which the tympanic membrane is 

 adjusted to enable it to receive vibrations of varying degrees of pitch. 



Function of the Ossicles. The function of the chain of bones is to 

 transmit the sound waves across the tympanic cavity to the internal ear. 

 The first of these bones, the malleus, being attached to the tympanic mem- 

 brane will take up the vibrations much more readily than if no membrane 

 intervened. Owing to the character of the articulations, when the handle 

 of the malleus is drawn inward, the position of the bones is so changed that 

 they form practically a solid rod, and are therefore much better adapted for the 

 transmission of molecular vibrations than if the articulations remained loose. 

 As the stapes bone is somewhat shorter than the malleus, its vibrations are 

 smaller than those of the tympanic membrane, and by this arrangement the 

 amplitude of the vibrations is diminished but their force increased. 



The Function of the Stapedius Muscle is, according to Henle, to 

 fix the stapes bone so as to prevent too great a movement from being com- 

 municated to it from the incus and transmitted to the perilymph. It may 

 be looked upon therefore, as a protective muscle. 



The Function of the Eustachian tube is to maintain a free communi- 

 cation between the cavity of the middle ear and naso-pharynx. The 

 pressure of air within and without the ear is thus equalized, and the vibra- 

 tions of the tympanic membrane permitted to attain their maximum ; one 

 of the conditions essential for the reception of sound waves. The impair- 

 ment in the acuteness of hearing which is caused by an unequal pressure of 

 the air in the middle ear can be shown : I. By closing the mouth and nose 

 and forcing air from the lungs through the Eustachian tube into the ear, 

 producing an increase in pressure. 2. By closing the nose and mouth, and 



